1. Field of the invention
The present invention relates, in general, to integrated circuits and methods for manufacturing thereof. More specifically, the invention relates to a charge coupled device (CCD) which has a lower gate electrode and an upper gate electrode and a method for manufacturing thereof.
2. Description of the prior art
In general, a CCD device has a plurality of lower gate electrodes and a plurality of upper gate electrodes. The lower gate electrodes are formed on a semiconductor substrate via a gate insulating film. The upper gate electrodes are also formed on the semiconductor substrate via a gate insulating film and are located between the lower gate electrodes. The upper gate electrodes and the upper gate insulating film are partly overhanging and adjacent to lower gate electrodes. During the operation of the CCD, relatively high voltage (ex. 9 or 12 volts) is appled across each of the lower gate insulting film and the upper gate insulating film.
On the other hand, the CCD is usually formed with MOS transistors, each having a gate insulating film, in the semiconductor substrate as an integrated circuit. In this case, the gate insulating film of the MOS transistor is formed by the same process as for either the lower gate insulating film or the upper gate insulating film. During the operation of the MOS transistors, relatively low voltage (e.g., 5 volts) is applied across the MOS gate insulating film.
In the prior art integrated circuit which has CCD and MOS transistors, if the MOS gate insulating film is made thinner with the aim of increasing the integration of MOS transistor portion according to the scaling rule, the lower gate insulating film or the upper gate insulating film also becomes thinner due to the aforementioned manufacturing process. In this type of CCD, it causes insulation breakdown in the lower gate insulating film or the upper gate insulating film. Thus, increasing the integration without causing insulation breakdown is difficult.
In addition, because the lower gate insulating film and the upper gate insulating film of the prior art CCD are formed in different manufacturing steps, it is difficult to equalize the characteristics of the two insulating films of the CCD and to improve the data transfer performance of the CCD.
In addition, because the upper gate electrode is overlapped with the lower gate electrode via the upper gate insulating film, insulation breakdown between lower and upper gate electrodes of the prior art CCD may occur.